The Northwestern University Center for Atom Probe Tomography (NUCAPT) specializes in high-resolution chemical imaging by three-dimensional atom probe tomography (APT), which is a combination of a field ion microscope (FIM) and a time-of-flight mass spectrometer. 3DAP microscopy involves the atom-by-atom dissection of a small volume of a material. Specimens are in the form of sharply-pointed needles, with an end radius less than 100 nm. These specimens are subjected to cryogenic temperature (~30 K) and high voltage (~10 kV), causing the atoms at the apex of the specimen tip to ionize and accelerate away from the positively-charged tip and towards a detector. The detector records both the ion's time-of-flight (and hence its mass-to-charge ratio - its chemical identity) and its impact position (and hence its original location on the tip surface prior to ionization). A typical 3DAP run records on the order of 500,000 ion times and positions. These data are analyzed using Apex, allowing for a reconstruction of all of the atomic positions within the analysis volume (typically of the order of 10 nm x 10 nm x 100 nm). Small, nanoscale preciptates and concentration fluctuations can be examined in real space, without data deconvolution and without comparison to standards. In this respect, the 3DAP represents the ultimate in nanoscale materials science chemical analysis.
Instruments: LEAP, IBS/e
Specimen Preparation: electropolishing, FIB
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